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A scuba set, originally just scuba, is any breathing apparatus that is entirely carried by an underwater diver and provides the diver with breathing gas at the ambient pressure. Scuba is an anacronym for self-contained underwater breathing apparatus. Although strictly speaking the scuba set is only the diving equipment that is required for providing breathing gas to the diver, general usage includes the harness or rigging by which it is carried and those accessories which are integral parts of the harness and breathing apparatus assembly, such as a jacket or wing style buoyancy compensator and instruments mounted in a combined housing with the pressure gauge. In the looser sense, scuba set has been used to refer to all the diving equipment used by the scuba diver, though this would more commonly and accurately be termed scuba equipment or scuba gear. Scuba is overwhelmingly the most common underwater breathing system used by recreational divers and is also used in professional diving when it provides advantages, usually of mobility and range, over surface-supplied diving systems and is allowed by the relevant legislation and code of practice.
Two basic functional variations of scuba are in general use: open-circuit-demand, and rebreather. In open-circuit demand scuba, the diver expels exhaled breathing gas to the environment, and each breath is delivered at ambient pressure, on demand, by a diving regulator which reduces the pressure from the storage cylinder. The breathing gas is supplied through a demand valve; when the diver inhales, they reduce the pressure in the demand valve housing, thus drawing in fresh gas. In rebreather scuba, the system recycles the exhaled gas, removes carbon dioxide, and compensates for the used oxygen before the diver is supplied with gas from the breathing circuit. The amount of gas lost from the circuit during each breathing cycle depends on the design of the rebreather and depth change during the breathing cycle. Gas in the breathing circuit is at ambient pressure, and stored gas is provided through regulators or injectors, depending on the design.
Within these systems, various mounting configurations may be used to carry the scuba set, depending on application and preference. These include: back mount, which is generally used for recreational scuba and for bailout sets for surface supplied diving; side-mount, which is popular for tight cave penetrations; sling mount, used for stage-drop sets; decompression gas and bailout sets where the main gas supply is back-mounted; and various non-standard carry systems for special circumstances.
The most immediate risk associated with scuba diving is drowning due to a failure of the breathing gas supply. This may be managed by diligent monitoring of remaining gas, adequate planning and provision of an emergency gas supply carried by the diver in a bailout cylinder or supplied by the Buddy diving, and the skills required to manage the gas sources during the emergency.
Open-circuit-demand scuba is a 1943 invention by the France Émile Gagnan and Jacques-Yves Cousteau, but in the English language Lambertsen's acronym has become common usage and the name Aqua-lung (often spelled "aqualung"), coined by Cousteau for use in English-speaking countries, has fallen into secondary use. As with radar, the acronym scuba has become so familiar that it is generally not capitalized and is treated as an ordinary noun. For example, it has been translated into the Welsh language as sgwba.
Although the term was originally an acronym, "scuba" is currently used to refer to the apparatus or the practice of diving using the apparatus, either alone as a common noun, or as an adjective in scuba set and scuba diving respectively. It is also used as an adjective referring to equipment or activity relating to diving using self-contained breathing apparatus.
Unlike other modes of diving, which rely either on Free-diving or on breathing gas supplied under pressure from the surface, scuba divers carry their own source of breathing gas, usually Air filter compressed air, allowing them greater freedom of movement than with an air line or diver's umbilical and longer underwater endurance than breath-hold. Scuba diving may be done recreationally or professionally in a number of applications, including scientific, military and public safety roles, but most commercial diving uses surface-supplied diving equipment for the main gas supply when this is practicable. Surface supplied divers may be required to carry scuba as an emergency breathing gas supply to get them to safety in the event of a failure of surface gas supply.
There are divers who work, full or part-time, in the recreational diving community as instructors, assistant instructors, divemasters and dive guides. In some jurisdictions the professional nature, with particular reference to responsibility for health and safety of the clients, of recreational diver instruction, dive leadership for reward and dive guiding is recognised and regulated by national legislation.
Other specialist areas of scuba diving include military diving, with a long history of military frogmen in various roles. Their roles include direct combat, infiltration behind enemy lines, placing mines or using a manned torpedo, bomb disposal or engineering operations. In civilian operations, many police forces operate police diving teams to perform "search and recovery" or "search and rescue" operations and to assist with the detection of crime which may involve bodies of water. In some cases search and rescue diving teams may also be part of a fire department, paramedical service or lifeguard unit, and may be classed as public safety diving.
There are also professional divers involved with the underwater environment, such as underwater photographers or underwater videographers, who document the underwater world, or scientific diving, including marine biology, geology, hydrology, oceanography and underwater archaeology.
The choice between scuba and surface supplied diving equipment is based on both legal and logistical constraints. Where the diver requires mobility and a large range of movement, scuba is usually the choice if safety and legal constraints allow. Higher risk work, particularly in commercial diving, may be restricted to surface supplied equipment by legislation and codes of practice.
The frequently quoted warning against holding one's breath on scuba is a gross oversimplification of the actual hazard. The purpose of the admonition is to ensure that inexperienced divers do not accidentally hold their breath while surfacing, as the expansion of gas in the lungs could over-expand the lung air spaces and rupture the alveoli and their capillaries, allowing lung gases to get into the pulmonary return circulation, the pleura, or the interstitial areas near the injury, where it could cause dangerous medical conditions. Holding the breath at constant depth for short periods with a normal lung volume is generally harmless, providing there is sufficient ventilation on average to prevent carbon dioxide buildup, and is done as a standard practice by underwater photographers to avoid startling their subjects. Holding the breath during descent can eventually cause lung squeeze, and may allow the diver to miss warning signs of a gas supply malfunction until it is too late to remedy.
Skilled open circuit divers can and will make small adjustments to buoyancy by adjusting their average lung volume during the breathing cycle. This adjustment is generally in the order of a kilogram (corresponding to a litre of gas), and can be maintained for a moderate period, but it is more comfortable to adjust the volume of the buoyancy compensator over the longer term.
The practice of shallow breathing or skip breathing in an attempt to conserve breathing gas should be avoided as it tends to cause a carbon dioxide buildup, which can result in headaches and a reduced capacity to recover from a breathing gas supply emergency. The breathing apparatus will generally increase dead space by a small but significant amount, and cracking pressure and flow resistance in the demand valve will cause a net work of breathing increase, which will reduce the diver's capacity for other work. Work of breathing and the effect of dead space can be minimised by breathing relatively deeply and slowly. These effects increase with depth, as density and friction increase in proportion to the increase in pressure, with the limiting case where all the diver's available energy may be expended on simply breathing, with none left for other purposes. This would be followed by a buildup in carbon dioxide, causing an urgent feeling of a need to breathe, and if this cycle is not broken, panic and drowning are likely to follow. The use of a low density inert gas, typically helium, in the breathing mixture can reduce this problem, as well as diluting the narcotic effects of the other gases.
Breathing from a rebreather is much the same, except that the work of breathing is affected mainly by flow resistance in the breathing loop. This is partly due to the carbon dioxide absorbent in the scrubber, and is related to the distance the gas passes through the absorbent material, and the size of the gaps between the grains, as well as the gas composition and ambient pressure. Water in the loop can greatly increase the resistance to gas flow through the scrubber. There is even less point in shallow or skip breathing on a rebreather as this does not even conserve gas, and the effect on buoyancy is negligible when the sum of loop volume and lung volume remains constant.
Both types of scuba set include a means of supplying air or other breathing gas, nearly always from a high pressure diving cylinder, and a harness to attach it to the diver. Most open-circuit scuba sets have a diving regulator to control the supply of breathing gas, and most rebreathers have a constant-flow injector, or an electronically controlled injector to supply fresh gas, but also usually have an automatic diluent valve (ADV), which functions in the same way as a demand valve, to maintain the loop volume during descent.
The essential subsystems of an open-circuit scuba set are;
The cylinder is usually worn on the back. "Twin sets" with two low capacity back-mounted cylinders connected by a high pressure manifold were more common in the 1960s than now for recreational diving, although larger capacity twin cylinders ("doubles") are commonly used by technical divers for increased dive duration and redundancy. At one time a firm called Submarine Products sold a sport air scuba set with three manifolded back-mounted cylinders. Cave and wreck penetration divers sometimes carry cylinders attached at their sides instead, allowing them to swim through more confined spaces.
This type of breathing set is sometimes called an aqualung. The word Aqua-Lung, which first appeared in the Cousteau-Gagnan patent, is a trademark, currently owned by Aqua Lung/La Spirotechnique.
All the stages of this type of regulator are in a large valve assembly mounted directly to the cylinder valve or manifold, behind the diver's neck. Two large bore corrugated rubber breathing hoses connect the regulator with the mouthpiece, one for supply and one for exhaust. The exhaust hose is used to return the exhaled air to the regulator, to avoid pressure differences due to depth variation between the exhaust valve and final stage diaphragm, which would cause a free-flow of gas, or extra resistance to breathing, depending on the diver's orientation in the water. In modern single-hose sets this problem is avoided by moving the second-stage regulator to the diver's mouthpiece. The twin-hose regulators came with a mouthpiece as standard, but a full-face diving mask was an option.
Modern regulators typically feature high-pressure ports for pressure sensors of dive-computers and submersible pressure gauges, and additional low-pressure ports for hoses for inflation of dry suits and BC devices.
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Most recreational scuba sets have a backup second-stage demand valve on a separate hose, a configuration called a "secondary", or "octopus" demand valve, "alternate air source", "safe secondary" or "safe-second". This arrangement was intended to reduce the problems of buddy breathing from a single demand valve and has become the standard in recreational diving. By providing a second demand valve the need to alternately breathe off the same mouthpiece when sharing air is eliminated. This reduces the stress on divers who are already in a stressful situation, and this in turn reduces air consumption during the rescue and frees the donor's hand.
Some diver training agencies recommend that a diver routinely offer their primary demand valve to a diver requesting to share air, and then switch to their own secondary demand valve. The idea behind this technique is that the primary demand valve is known to be working, and the diver donating the gas is less likely to be stressed or have a high carbon dioxide level, so has more time to sort out their own equipment after temporarily suspending the ability to breathe. In many instances, panicked divers have grabbed the primary regulators out of the mouths of other divers, so changing to the backup as a routine reduces stress when it is necessary in an emergency.
In technical diving donation of the primary demand valve is commonly the standard procedure, and the primary is connected to the first stage by a long hose, typically around 2 m, to allow gas sharing while swimming in single file in a narrow space as might be required in a cave or wreck. In this configuration the secondary is generally held under the chin by a loose bungee loop around the neck, supplied by a shorter hose, and is intended for backup use by the diver donating gas. The backup regulator is usually carried in the diver's chest area where it can be easily seen and accessed for emergency use. It may be worn secured by a breakaway clip on the buoyancy compensator, plugged into a soft friction socket attached to the harness, secured by sliding a loop of the hose into the shoulder strap cover of a jacket style BC, or suspended under the chin on a break-away bungee loop known as a necklace. These methods also keep the secondary from dangling below the diver and being contaminated by debris or snagging on the surroundings. Some divers store it in a BC pocket, but this reduces availability in an emergency.
Occasionally, the secondary second-stage is combined with the inflation and exhaust valve assembly of the buoyancy compensator device. This combination eliminates the need for a separate low pressure hose for the BC, though the low pressure hose connector for combined use must have a larger bore than for standard BC inflation hoses, because it will need to deliver a higher flow rate if it is used for breathing. This combination unit is carried in the position where the inflator unit would normally hang on the left side of the chest. With integrated DV/BC inflator designs, the secondary demand valve is at the end of the shorter BC inflation hose, and the donor must retain access to it for buoyancy control, so donation of the primary regulator to help another diver is essential with this configuration. The secondary demand valve is often partially yellow in color, and may use a yellow hose, for high visibility, and as an indication that it is an emergency or backup device.
When a side-mount configuration is used, the usefulness of a secondary demand valve is greatly reduced, as each cylinder will have a regulator and the one not in use is available as a backup. This configuration also allows the entire cylinder to be handed off to the receiver, so a long hose is also less likely to be needed.
Some diving instructors continue to teach buddy-breathing from a single demand valve as an obsolescent but still occasionally useful technique, learned in addition to the use of the backup DV, since availability of two second stages per diver is now assumed as standard in recreational scuba.
The Russian Kriolang (from Greek cryo- (= "frost" taken to mean "cold") + English "lung") was copied from Jordan Klein's "Mako" cryogenic open-circuit scuba. and were made until at least 1974. It would have to be filled a short time before use.
There are two main variants of rebreather – semi-closed circuit rebreathers, and fully closed circuit rebreathers, which include the subvariant of oxygen rebreathers. Oxygen rebreathers have a maximum safe operating depth of around , but several types of fully closed circuit rebreathers, when using a helium-based diluent, can be used deeper than . The main limiting factors on rebreathers are the duration of the carbon dioxide scrubber, which is generally at least 3 hours, increased work of breathing at depth, reliability of gas mixture control, and the requirement to be able to safely bail out at any point of the dive.
Rebreathers are generally used for scuba applications, but are also occasionally used for or for surface supplied diving.
The possible endurance of a rebreather dive is longer than an open-circuit dive, for similar weight and bulk of the set, if the set is bigger than the practical lower limit for rebreather size, and a rebreather can be more economical when used with expensive gas mixes such as heliox and trimix, but this may require a lot of diving before the break-even point is reached, due to the high initial and running costs of most rebreathers, and this point will be reached sooner for deep dives where the gas saving is more pronounced.
Cylinder working pressure will vary according to the standard of manufacture, generally ranging from up to .
An aluminium cylinder is thicker and bulkier than a steel cylinder of the same capacity and working pressure, as suitable aluminium alloys have lower tensile strength than steel, and is more buoyant although actually heavier out of the water, which means the diver would need to carry more ballast weight. Steel is also more often used for high pressure cylinders, which carry more air for the same internal volume.
The common method of blending nitrox by partial pressure requires that the cylinder is in "oxygen service", which means that the cylinder and cylinder valve have had any non-oxygen-compatible components replaced and any contamination by combustible materials removed by cleaning. Diving cylinders are sometimes colloquially called "tanks" "scuba tanks", "bottles" or "flasks", and some of these may be equivalent to the correct term in other languages although the proper technical term for them is "cylinder" or "scuba cylinder".
Rebreather divers and some open-circuit scuba divers carry extra for bailout in case the main breathing gas supply is used up or malfunctions. If the bailout cylinder is small, they may be called "pony cylinders". They have their own diving regulator and mouthpieces, and are technically distinct extra scuba sets. In technical diving, the diver may carry different equipment for different phases of the dive. Some breathing gas mixes, such as trimix, may only be used at depth, and others, such as pure oxygen, may only be used during decompression stops in shallow water. The heaviest cylinders are generally carried on the back supported by a backplate while others are side slung from strong points on the harness.
This style of harness was originally used in this simple form, but is currently more usually used with a back inflation wing type buoyancy compensator sandwiched between the cylinder and the backplate.
A more complex but still minimalist system is a webbing harness with shoulder straps, waist belt and crotch strap, supporting a variety of sliders and D-rings for attachment of cylinders and accessories, with or without integrated weighting or separate weight belts, and with or without a back mounted buoyancy compensator, which may be attached to the harness, or directly to the diver. Cylinders are usually attached to a shoulder or chest D-ring and waist belt D-ring on each side.
Many closed circuit rebreathers use advanced electronics to monitor and regulate the composition of the breathing gas.
Mouthpiece retaining straps have also been used with open circuit scuba, both on single hose and twin hose units. They were quite common as original manufacturer's equipment in the 1960s, when they were usually called neck straps, but at some stage lost popularity. The technical diving community later developed a similar functioning accessory generally referred to as a or bungee necklace, an aftermarket or home made elastic loop, generally used to hold the secondary demand valve in a position under the chin where it can be accessed hands-free by tilting the head. It can also be used to hold a primary demand valve in the same way, which will keep it in close proximity to the face if accidentally dropped or dislodged, making it often possible to recover hands-free. This type of retainer does not necessarily keep the mouthpiece in place in the diver's mouth or maintain a seal if the diver loses consciousness, as the fit is not always adjustable. The bungee necklace can be pulled away from the neck with a moderate force, causing the rubber mouthpiece to pop out of the retaining loop.
Designing an adequate diffuser for a rebreather is much easier than for open-circuit scuba, as the gas flow rate is generally much lower. An open-circuit diffuser system called the "scuba muffler" was prototyped by Eddie Paul in the early 1990s for underwater photographers John McKenney and Marty Snyderman; the prototype had two large filter stones mounted on the back of the cylinder with a hose connected to the exhaust ports of the second-stage regulator. The filter stones were mounted on a hinged arm to float above the diver, to set up a depth-pressure-differential suction effect to counteract the extra exhalation pressure needed to breathe out through the diffuser. The scuba muffler was claimed to cut the exhalation noise by 90%. Closed circuit rebreathers proved more useful in letting divers get near sharks.
An open-circuit diver whose breathing rate at the surface (atmospheric pressure) is 15 litres per minute will consume 3 × 15 = 45 litres of gas per minute at 20 metres. (20 × 15 L/min = 45 L/min). If an 11-litre cylinder filled to 200 bar is to be used until there is a reserve of 17% there is (83% × 200 × 11) = 1826 litres available. At 45 L/min the dive at depth will be a maximum of 40.5 minutes (1826/45). These depths and times are typical of experienced recreational divers leisurely exploring a coral reef using standard 200 bar "aluminum 80" cylinders as may be rented from a commercial recreational diving operation in most tropical island or coastal resorts.
In practice, dive times for rebreathers are more often influenced by other factors, such as hypothermia and the need for safe ascent (see Decompression (diving)), and this is generally also true for large-capacity open-circuit sets.
Scuba is safety-critical equipment, as some modes of failure can put the user at immediate risk of death by drowning, and a catastrophic failure of a scuba cylinder can instantly kill or severely injure persons in the vicinity. Open circuit scuba is considered highly reliable if correctly assembled, tested, filled, maintained and used, and the risk of failure is fairly low, but high enough that it should be considered in dive planning, and where appropriate, precautions should be taken to allow appropriate response in case of a failure. Mitigation options depend on the circumstances and mode of failure.
A large number of cylinders, hoses and fittings passing through the water tends to increase hydrodynamic drag, reducing swimming efficiency.
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By the turn of the twentieth century, two basic architectures for underwater breathing apparatus had been pioneered; open-circuit surface supplied equipment where the diver's exhaled gas is vented directly into the water, and closed-circuit breathing apparatus where the diver's carbon dioxide is filtered from unused oxygen, which is then recirculated. Closed circuit equipment was more easily adapted to scuba in the absence of reliable, portable, and economical high pressure gas storage vessels. By the mid twentieth century, high pressure cylinders were available and two systems for scuba had emerged: open-circuit scuba where the diver's exhaled breath is vented directly into the water, and closed-circuit scuba where the carbon dioxide is removed from the diver's exhaled breath which has oxygen added and is recirculated. Oxygen rebreathers are severely depth limited due to oxygen toxicity risk, which increases with depth, and the available systems for mixed gas rebreathers were fairly bulky and designed for use with diving helmets. The first commercially practical scuba rebreather was designed and built by the diving engineer Henry Fleuss in 1878, while working for Siebe Gorman in London. His self contained breathing apparatus consisted of a rubber mask connected to a breathing bag, with an estimated 50–60% oxygen supplied from a copper tank and carbon dioxide scrubbed by passing it through a bundle of rope yarn soaked in a solution of caustic potash, the system giving a dive duration of up to about three hours. This apparatus had no way of measuring the gas composition during use. During the 1930s and all through World War II, the British, Italians and Germans developed and extensively used oxygen rebreathers to equip the first Frogman. The British adapted the Davis Submerged Escape Apparatus and the Germans adapted the Dräger submarine escape rebreathers, for their frogmen during the war. In the U.S. Major Christian J. Lambertsen invented an underwater free-swimming oxygen rebreather in 1939, which was accepted by the Office of Strategic Services. In 1952 he patented a modification of his apparatus, this time named SCUBA, (an acronym for "self-contained underwater breathing apparatus"), which became the generic English word for autonomous breathing equipment for diving, and later for the activity using the equipment. After World War II, military frogmen continued to use rebreathers since they do not make bubbles which would give away the presence of the divers. The high percentage of oxygen used by these early rebreather systems limited the depth at which they could be used due to the risk of convulsions caused by acute oxygen toxicity.
Although a working demand regulator system had been invented in 1864 by Auguste Denayrouze and Benoît Rouquayrol, the first open-circuit scuba system developed in 1925 by Yves Le Prieur in France was a manually adjusted free-flow system with a low endurance, which limited the practical usefulness of the system. In 1942, during the German occupation of France, Jacques-Yves Cousteau and Émile Gagnan designed the first successful and safe open-circuit scuba, known as the Aqua-Lung. Their system combined an improved demand regulator with high-pressure air tanks. This was patented in 1945. To sell his regulator in English-speaking countries Cousteau registered the Aqua-Lung trademark, which was first licensed to the U.S. Divers company, and in 1948 to Siebe Gorman of England, Siebe Gorman was allowed to sell in Commonwealth countries, but had difficulty in meeting the demand and the U.S. patent prevented others from making the product. The patent was circumvented by Ted Eldred of Melbourne, Australia, who developed the single-hose open-circuit scuba system, which separates the first stage and demand valve of the pressure regulator by a low-pressure hose, puts the demand valve at the diver's mouth, and releases exhaled gas through the demand valve casing. Eldred sold the first Porpoise Model CA single hose scuba early in 1952.
Early scuba sets were usually provided with a plain harness of shoulder straps and waist belt. The waist belt buckles were usually quick-release, and shoulder straps sometimes had adjustable or quick release buckles. Many harnesses did not have a backplate, and the cylinders rested directly against the diver's back. Early scuba divers dived without a buoyancy aid. In an emergency they had to jettison their weights. In the 1960s adjustable buoyancy life jackets (ABLJ) became available, which can be used to compensate for loss of buoyancy at depth due to compression of the neoprene wetsuit and as a lifejacket that will hold an unconscious diver face-upwards at the surface, and that can be quickly inflated. The first versions were inflated from a small disposable carbon dioxide cylinder, later with a small direct coupled air cylinder. A low-pressure feed from the regulator first-stage to an inflation/deflation valve unit an oral inflation valve and a dump valve lets the volume of the ABLJ be controlled as a buoyancy aid. In 1971 the stabilizer jacket was introduced by ScubaPro. This class of buoyancy aid is known as a buoyancy control device or buoyancy compensator.
A backplate and wing is an alternative configuration of scuba harness with a buoyancy compensation bladder known as a "wing" mounted behind the diver, sandwiched between the backplate and the cylinder or cylinders. Unlike stabilizer jackets, the backplate and wing is a modular system, in that it consists of separable components. This arrangement became popular with cave divers making long or deep dives, who needed to carry several extra cylinders, as it clears the front and sides of the diver for other equipment to be attached in the region where it is easily accessible. This additional equipment is usually suspended from the harness or carried in pockets on the exposure suit. Sidemount is a scuba diving equipment configuration which has basic scuba sets, each comprising a single cylinder with a dedicated regulator and pressure gauge, mounted alongside the diver, clipped to the harness below the shoulders and along the hips, instead of on the back of the diver. It originated as a configuration for advanced cave diving, as it facilitates penetration of tight sections of cave as, sets can be easily removed and remounted when necessary. The configuration allows easy access to cylinder valves, and provides easy and reliable gas redundancy. These benefits for operating in confined spaces were also recognized by divers who made wreck diving penetrations. Sidemount diving has grown in popularity within the technical diving community for general decompression diving, and has become a popular specialty for recreational diving.
Technical diving is recreational scuba diving that exceeds the generally accepted recreational limits, and may expose the diver to hazards beyond those normally associated with recreational diving, and to greater risks of serious injury or death. These risks may be reduced by appropriate skills, knowledge and experience, and by using suitable equipment and procedures. The concept and term are both relatively recent advents, although divers had already been engaging in what is now commonly referred to as technical diving for decades. One reasonably widely held definition is that any dive in which at some point of the planned profile it is not physically possible or physiologically acceptable to make a direct and uninterrupted vertical ascent to surface air is a technical dive. The equipment often involves breathing gases other than air or standard nitrox mixtures, multiple gas sources, and different equipment configurations. Over time, some equipment and techniques developed for technical diving have become more widely accepted for recreational diving.
The challenges of deeper dives and longer penetrations and the large amounts of breathing gas necessary for these dive profiles and ready availability of oxygen sensing cells beginning in the late 1980s led to a resurgence of interest in rebreather diving. By accurately measuring the partial pressure of oxygen, it became possible to maintain and accurately monitor a breathable gas mixture in the loop at any depth. In the mid 1990s semi-closed circuit rebreathers became available for the recreational scuba market, followed by closed circuit rebreathers around the turn of the millennium. Rebreathers are currently (2018) manufactured for the military, technical and recreational scuba markets.
See also
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